“JUSTIN has agreed to cut all tariffs and all trade barriers between Canada and the United States,” claimed President Donald Trump to laughter on June 8th, at the G7 summit in Quebec. The next day, in apparent seriousness, Mr Trump—who has slapped tariffs and quotas on imports of aluminium and steel from all the G7 countries, and others—called for unfettered trade within the group: “No tariffs, no barriers. That’s the way it should be.”

Over the next two days a more familiar Mr Trump reappeared. After Mr Trudeau said, at a post-summit press conference, that Canada would not be pushed around, he fired off a barrage of tweets calling him “very dishonest & weak”. He blasted Europe too. And he tweeted: “Sorry, we cannot let our friends, or enemies, take advantage of us on Trade anymore.”

Suspend disbelief and suppose that Mr Trump’s offer of a barrier-free world is serious. He may want to tear down tariffs and quotas out of a yearning for open markets and lower prices for...